Start Capacitor

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Frequently Asked Questions...

What Size Resistor to bleed off start capacitor?

I need a resistor to bleed off a start copacitor; this is the specs on it,,,,,Start Capacitor,270-324 MFD,220-250 VAC ..

I was told a 500 ohm 10 w would do the trick but seem hard to find, is there another size I can use that can be found at maybe radio shack or not?


Best Answer...

Answer:

I don't think a start capacitor needs a bleeder resister because it is never charged to any DC level. I think a start capacitor is just used to shift the phase of the ac current in one winding of the motor with respect to the phase of the ac current in another winding of the motor in order to get enough torque to start the motor. The start capacitor is switched out of the circuit when the motor reaches a certain rpm. I don`t remember the details and may be wrong about the whole thing.

I would recommend that you run the motor until it gets up to speed without a bleeder resistor then disconnect power to it and immediately measure the DC voltage across the capacitor to see for your self whether or not you actually need a bleeder resistor.

If you do require a bleeder resistor it`s resistance value would need to be high compared to the impedance of the start capacitor other wise you will shunt out the effects of the start capacitor and the motor will not start. Of course the higher the resistance value of the bleeder resistor is the smaller the wattage requirement of the resistor will be but the longer it will take for the capacitor to discharge. If you need a bleeder resistor at all I am sure you can use one with a value much greater than 500 ohms which will allow you to use one with a power rating of much less than 10 watts.